1. Field of the Invention
The present invention relates to a vehicular air conditioner using a heat pump, which is installed in vehicles such as automobiles.
2. Description of the Related Art
Recently, there has been an increasing demand for introducing a low-pollution vehicle and alternative energy vehicle, accompanied with demands for improving the air environment and environmental problems. When the energy source is replaced by natural gas, this is basically a change only in the fuel, and there is no need to change the basic structure of an air conditioning apparatus (hereinafter referred to as an air conditioner), since there is still an internal combustion engine (hereinafter referred to as an engine).
If however a conventional air conditioner is used as is, in an electric vehicle or a hybrid vehicle (using both an electric motor and an engine as a drive source) which is one of the strong candidates for the alternative energy vehicle, it is necessary to reconsider the heat source during the heating operation and the compressor drive source during the cooling operation.
That is to say, a problem arises in that during the heating operation, with electric vehicles there is no engine cooling water to serve as the heating source as in the conventional vehicles, and in hybrid vehicles there is a motor travelling mode in which the engine is stopped and the vehicle is driven only by the electric motor, and hence sufficient warm water cannot be obtained.
Moreover, during the cooling operation, the drive source for the compressor cannot rely only upon the engine as with conventional vehicles, and another drive source must be provided. For example, in the case of a hybrid vehicle, there is a motor travelling mode in which the vehicle is driven only by the electric motor, or even if the vehicle is driven by the engine, at the time of stopping, the engine is stopped so as not to run in idle. Hence stable operation of the air conditioner is not possible when only the engine is used as the drive source for the compressor.
From this background, a heat pump type air conditioner used in household cooling/heating air conditioners has been adopted as the air conditioner installed in vehicles such as the electric vehicles and hybrid vehicles.
FIG. 4 shows a schematic structural diagram of a conventional vehicular air conditioner using a heat pump. In the figure, numeral 1 denotes an indoor heat exchanger, 2 denotes a compressor unit, 3 denotes an outdoor heat exchanger, and 4 denotes a fan for drawing in outside air. In this case, the outdoor heat exchanger 3 is installed inside of an engine compartment together with the compressor unit 2 and the like. By activating the fan 4 for drawing in outside air 4, the outside air can be drawn into the engine compartment.
With the above described conventional construction, a refrigerant is circulated as described below to effect cooling and heating in the vehicle cabin.
The refrigerant during the heating operation circulates in a clockwise direction, as shown by the solid arrow in the figure. The refrigerant which is changed to a high-temperature and high-pressure gas by the compressor in the compressor unit 2 is sent to the indoor heat exchanger 1 to exchange heat with the air outside the vehicle (the outside air) or with the air inside the vehicle (the inside air). As a result, the outside air or the inside air (hereinafter referred to as intake air) becomes hot air by absorbing heat from the high-temperature and high-pressure gas refrigerant, and at the same time, the high-temperature and high-pressure gas refrigerant looses heat to be changed into a condensate, and becomes a high-temperature and high-pressure liquid refrigerant.
Subsequently, the high-temperature and high-pressure liquid refrigerant passes through the compressor unit 2 where it is expanded to become a low-temperature and low-pressure liquid refrigerant and is sent to the outdoor heat exchanger 3. In the outdoor heat exchanger 3, the low-temperature and low-pressure liquid refrigerant draws up heat from the outside air and is evaporated and gasified to become a low-temperature and low-pressure gas refrigerant. This low-temperature and low-pressure gas refrigerant is again sent to the compressor unit 2 and compressed, to become a high-temperature and high-pressure gas. Hereafter, the above described process is repeated.
That is to say, during the heating operation, the outdoor heat exchanger 3 functions as an evaporator, and the indoor heat exchanger 1 functions as a condenser.
If such a heating operation is continued, frost attaches to the outdoor heat exchanger 3, and sufficient heat exchange cannot be effected. Therefore, if a predetermined defrost condition is satisfied, a defrost operation is performed to melt the frost by switching the direction of flow of the refrigerant, that is, by making the refrigerant flow as in a cooling operation described later. With such a defrost operation, the outdoor heat exchanger 3 which functions as an evaporator during the heating operation functions as a condenser, and hence the frost can be melt by receiving heat from the refrigerant.
The refrigerant during the cooling/dehumidifying operation circulates in the counterclockwise direction as shown by the broken line arrow in the figure. The refrigerant which is changed to a high-temperature and high-pressure gas by the compressor in the compressor unit 2 is sent to the outdoor heat exchanger 3 to exchange heat with the outside air. As a result, the refrigerant gives up heat to the outdoor air and is changed into a condensate, becoming a high-temperature and high-pressure liquid refrigerant. The refrigerant which becomes the high-temperature and high-pressure liquid refrigerant as described above passes through a throttling resistance in the compressor unit 2 to become a low-temperature and low-pressure liquid refrigerant, and is then sent to the indoor heat exchanger 1.
Subsequently, the low-temperature and low-pressure liquid refrigerant absorbs heat from the intake air in the indoor heat exchanger 1 to cool the air. Hence cool air can be supplied to the vehicle cabin, and at the same time, the refrigerant itself is evaporated and gasified to become a low-temperature and low-pressure gas refrigerant. The refrigerant which becomes the low-temperature and low-pressure gas refrigerant is again sent to the compressor in the compressor unit 2, and compressed to become a high-temperature and high-pressure gas. Hereafter, the above described process is repeated. That is, during the cooling operation, the indoor heat exchanger 1 functions as an evaporator, and the outdoor heat exchanger 3 functions as a condenser.
With the above described conventional vehicular air conditioner using a heat pump, it is desirable effectively utilize the waste heat from the drive source (engine, electric motor, and the like) during the heating operation.
That is to say, under a situation of low outside air temperature, there is a case where it may be difficult to draw up a sufficient quantity of heat from the outside air using the heat pump, resulting in a problem in that the heating capacity of the air conditioner is not sufficient. For example, the temperature is not increased as desired, or it takes a long time to reach a desired temperature. Therefore, it is highly desirable to improve the heating ability by drawing up heat from engine waste heat or the like, which has been conventionally discarded, using the heat pump, and effectively utilizing this heat in the heating operation of the air conditioner, to thereby improve the marketability of the vehicular air conditioner.
Moreover, the above described defrost operation executes a cooling operation under situations where a heating operation is required. Therefore, from the viewpoint of providing a comfortable vehicle cabin environment to passengers, preferably the frequency of the defrost operations should be a minimum and these should be terminated within a short period of time. Furthermore, when the defrost operation is initiated or terminated, a four way valve is operated to change the flow direction of the refrigerant. At this time, since the high pressure side and the low pressure side is changed over, an abnormal sound occurs. The occurrence of this abnormal sound is undesirable for a vehicular air conditioner, and it is desired to take measures against such a problem. Solving such a problem during the defrost operation is a key issue for improving the marketability of the vehicular air conditioner.